This invention relates to a valve operating mechanism for an engine and more particularly to an improved cam and follower profile for operating the intake and exhaust valves of an internal combustion engine.
In many forms of engines, the poppet vales are opened by a cam and follower mechanism that is comprised of a rotating cam which is carried on a cam shaft and which is driven in timed relationship to the engine output shaft. The cam generally operates the actuated valve through a follower type mechanism which may be of the thimble tappet type in connection with direct actuation or through a rocker arm in connection with indirect actuation. The valve is urged toward its closed position by some form of spring arrangement which frequently employs mechanical springs that act on the valve and/or rocker arm.
This type of mechanism has some disadvantages. First, because of the fact that the reciprocating movement of the valves is accomplished by translating a rotary motion into such motion, there is wear between the cam and follower surfaces. Also, the operation is such that inertial and other loading can cause a loss of contact between the cam lobe and its follower. This results in a condition known as "valve float". Valve float generally occurs at higher engine speeds and this condition generally is one of those factors that determine the maximum permissible engine speed.
When valve float occurs, substantial problems may arise and, therefore, the engine must be operated at low enough speeds so that as to avoid valve float. This reduces the potential maximum power output of the engine, as should be readily apparent.
Because the angular duration of crankshaft rotational movement during which the valves may be held open is limited, it is also desirable to control the valve opening in such a way that the valve is opened and closed rather rapidly and held in its maximum opened position for a fairly substantial duration of crankshaft rotation in order to improve the breathing capabilities to the engine. However, the stresses and wear aforenoted limit the maximum accelerations that can be enjoyed to open the valve and also, the conditions which are necessary to maintain the valve in its open position during engine running also can effect valve float.
These problems may be understood at least in part by reference to FIG. 1 which is a graphical view showing certain conditions during the opening and closing of a poppet type valve which may comprise either an intake valve or an exhaust valve for the engine. These curves are typical for the valve operation regardless of whether the valve is directly or indirectly operated.
FIG. 1 is a graphical view that shows the angular rotation of the cam shaft or cam on the ordinate and the degree of motion of the associated valve and certain characteristics of its motion such as its acceleration and rate of change of acceleration (jerk) on the abscissas. In this graphical view, it is assumed that the angular rotational velocity of the cam shaft and cam is constant as it generally is in an engine.
It will be seen that during the opening and closing cycle of the valve, the valve lift follows the curve Y. In connection with this, the cam lobe has a base circle or heel portion that has no lift and which has a constant radius R.sub.o that is centered on the cam shaft axis. The lift portion of the lobe is configured, as shown in curve Y so as to cause the valve to open and the opening follows a generally parabolic configuration of increase in lift amount after leaving the heal portion. As the opening continues, there is an inverse parabolic decrease in the lift amount until fully open. Closure occurs in a mirror image fashion with an inverse parabolic decrease in the lift amount upon initial closing. At the end of the closure, the decrease in lift amount again follows a parabolic curve whereupon the valve again is seated in its closed position.
This type of lift characteristic gives a valve acceleration component shown by the curve Y' that causes the valve acceleration to increase rapidly during the initial lift portion and then gradually decrease through the time when the valve is fully opened. At this time, the valve acceleration then turns negative and follows the a mirrored curve during the closing portion. This negative acceleration decreases rather abruptly when the tip of the ramp portion of the cam lobe is reached and continues to decelerate rapidly until the valve is fully closed.
The remaining curve of FIG. 1, which is labeled as Y" which represents the jerk forces on valve. These forces are related to the differential of the acceleration curve. As may be seen, there is a very rapidly increasing jerk force during the initial acceleration opening of the valve which falls off rather rapidly and then goes negative during the time when the valve is opening and begins to close in its parabolic curve configuration. The maximum negative jerk force occurs at the time when the valve is fully opened.
The jerk force is related to the actual bearing force between the cam surface and the follower or valve. Thus, when this value is low, there is a condition when there becomes a likelihood that the valve and/or follower will not follow the motion of the cam and cause the valve floating problem which is clearly undesirable.
The actual loading on the cam surface is the sum of the load expressed by action of the valve spring and the inertial force expressed by the product of the inertial mass of the actuated components (valve, portion of the valve spring and follower) and the acceleration of these components.
The stress on the surfaces of the cam and follower is proportional to the load acting on the surfaces and their effective area. The area of the cam surface is related to the inverse proportion of the square root of its radius of curvature.
With conventional cam profiles, when running in the low and medium speed range where the rotational speed of the cam shaft is low and there is a small influence of the acceleration, the maximum stress occurs in the maximum lift portion of the cam profile where the resilient force of the spring acting on the cam surfaces is at a maximum. At this time, the valve spring is at its maximum compression or deflection. Thus, with a conventional engine as utilized in automotive practice operated under low and medium speeds, the high stresses tend to cause a greater amount of wear and decreases the life or durability of the valve mechanism.
In addition to the high stresses, particular under the low and medium speeds with conventional cam constructions, the configuration of the tip of the lobe portion also tends to promote or, said another way, increase the likelihood of valve float. The valve spring tends to create a force on the valve follower that urges it into contact with the cam nose. However, as seen in FIG. 1, the negative acceleration at this point tends to cause separation due to the initial forces and thus, the follow up behavior between the cam nose and the follower at high speed is sacrificed and valve float can occur.
It is, therefore, a principal object of this invention to provide an improved cam profile for operating the poppet valves of an engine wherein the stress on the cam lobe surface is reduced and the loading under maximum lift conditions between the cam and the follower is increased so as to avoid float and thus, permit operation at higher engine speeds.
It is, thus, the principal object of this invention to provide an improved cam profile for operating the valve of a reciprocating engine wherein the engine can be operated at higher speeds and also wherein durability of the valve components and specifically the cam and follower are improved.